Real-time and label-free sensing of charged biomolecules, chemicals and ions is useful in many applications such as toxin detection, disease diagnosis, and drug screening. For highly specific biomolecular detection, various optical sensing approaches have been developed that are categorized as either fluorescence-based detection or label-free detection. Although fluorescence-based detection permits, in some instances, a sensitivity down to a single molecule, an imprecise number of fluorophores per molecule limits the quantitative nature of these assays. Label-free optical methods including refractive index change, optical absorption, and Raman spectroscopic detection are relatively simple but are hampered by scalability and sensitivity.
Highly sensitive MEMS-based mechanical biosensors have been investigated that employ either an optical or electrical detection scheme to track the cantilever displacement upon specific binding between the biofunctionalized receptor layer and target molecules. However, to increase sensitivity, the cantilever dimension must be reduced into the nanoscale which in turn compromises the viability of optical detection due to diffraction during the tip focusing. Although integrated piezoresistive cantilever biosensors eliminate an optical detection component, the inherent detection of the quasi-static surface-layer by these devices induced stress, and, prevents these devices from tracking the stochastic nature of affinity-based interactions, which is on the order of nanoseconds or microseconds. This limitation is especially true when the target molecule is in the presence of many weakly bound species.
Quasi-1-D semiconductor nanowires are uniquely suitable for high sensitivity label-free detection applications. Their microscale to nanoscale volumes and large surface to volume ratio are respectively favorable for bulk detection, e.g., radiation, and surface sensing, for example, to detect biochemical molecules. Semiconductor nanowires have previously been configured as substrate-gated FET channels. Exposed Si nanowires atop an insulator layer have exhibited a limit of detection (LOD) of <100 fM for immunoglobulins and ˜10 fM for DNA. These sensing nanowires have also been integrated into detection systems with microfluidic modules.